The invention relates to a film-type evaporator for thickening liquid products with a container having a vapor outlet, a drive shaft introduced into the latter from above for a rotating rotor with at least one conically upwardly widening heating surface, onto whose inside the product is fed in the vicinity of the smaller diameter and the product spreads in film-like manner towards the larger diameter and is fed there as a concentrate into the container, and on whose outside is guided the heating medium supplied and removed through the hollow drive shaft.
Film-type evaporators having the aforementioned construction operate according to the centrifugal principle, in that the product is fed onto rapidly rotating, axially symmetrical heating surfaces, which are heated with liquid or vaporous heat carriers. As a result of their conical or tapered construction, said heating surfaces have a more or less pronounced slope or taper with respect to a radial plane, so that the product fed close to the axis spreads in film-like manner to the outside. Within the film there is largely a laminar flow, which once again leads to a purely surface evaporation. As a result of the high heat transmission coefficient (kcal/m.sup.2 /h.degree. C.), the circulation residence time of the product on the heating surface can be kept short, i.e. a few seconds. Therefore the film-type evaporator makes it possible to evaporate the product in a very careful manner. Therefore in the case of many heat-sensitive products, as well as highly viscous products (up to 50,000 cP), it often constitutes the only distillation or concentration possibility.
Another advantage of thin film evaporation with rotating heating surfaces results from the fact that the liquid film is always pressed onto the heating surface due to the centrifugal forces. As a result bubble and froth formation is suppressed, so that film-type evaporators are particularly suitable for the processing of products having a froth or foam formation tendency.
Single-stage or multistage film-type evaporators exist. With the multistage construction, the concentrate obtained on the outer circumference of the heating surface in the first evaporation stage is fed onto at least one further heating surface of a second evaporation stage, where it undergoes further concentration.
Very high evaporation rates, which not infrequently exceed 90% of the liquid content in the starting product, cannot be achieved in the case of single-stage evaporation. However, with a multistage construction, a considerable constructional expenditure occurs and a significant contribution thereto is made by the high dynamic loads resulting from high speeds and the necessarily thin-walled heating surfaces. Thus, in such cases, it is still very frequent to work with closed reboilers.
The problem of the invention is to so construct a film-type evaporator that maximum evaporation rates can be achieved with minimum constructional expenditure.
On the basis of the film-type evaporator defined hereinbefore, this problem is solved in that in the vicinity of its larger diameter, the rotor is connected by means of a spider, which has at least one hollow arm in each case for the feed and removal of the heating medium, to the drive shaft, and in the vicinity of its smaller diameter is traversed by a conical immersion tube rotating with the rotor and which is immersed with its smaller diameter in the product or concentrate in the lower part of the container and on its larger diameter is connected to the drive shaft of the rotor and has there an overflow edge feeding the product or concentrate onto the heating surface.
Apart from the rotary, conical heating surface, the inventively constructed film-type evaporator has a corotating pump in the form of an immersion tube pump, which can suck in the product or the concentrate collecting in the lower part of the container, or a mixture thereof and supply same to the heating surface. Thus, the evaporator can operate in the singlepass process or in the circulation process. In the case of the former, the product is sucked out of the lower part of the container by means of the immersion tube pump and fed onto the heating surface on which it is distributed in the form of a film. The concentrate collecting on the outer circumference of the heating surface is sprayed into the container and collects in its lower part from where it can be removed. With a circulation process in order to bring about a high concentration or thickening, the concentrate collecting in the lower part is supplied to the immersion tube pump and by the latter back to the heating surface. A specific filling level in the lower part will be ensured. Through a plurality of circulations the concentration can be brought to the desired level. A continuous operation is also possible, in that a specific small concentrate quantity is constantly removed and replaced by a fresh product quantity adapted thereto and to the evaporated quantity.
According to a preferred construction, the immersion tube engages with its smaller diameter in an upwardly open chamber arranged in the lower part of the container and which is connected to a product feed line and can optionally be connected to a concentrate drain of the container.
In this embodiment a specific filling level for the supplied product or the returned concentrate can be set in the chamber and therefore on the suction side of the immersion tube, so as in this way to ensure that the same evaporation ratios always exist on the heating surface. The chamber also ensures a separation of the concentrate collecting in the lower part from the liquid fed in (fresh product or preconcentrate).
If the immersion tube is axially adjustable on the drive shaft, there is a simple possibility for regulating the quantity supplied to the heating surface in the case of a given rotor speed or for maintaining the quantity constant when the speed varies (modification of the product residence time on the heating surface).
According to a further preferred development of the invention, a further conical, outer immersion tube is provided, which spacedly surrounds the chamber in the vicinity of its smaller diameter and is immersed in the concentrate collecting in the lower part and which in the vicinity of its larger diameter is connected to the rotor and has an overflow edge positioned above the concentrate level.
The additional outer immersion tube acts as a type of mixer for the concentrate collecting in the lower part, in that it ensures a constant circulation. The concentrate sprayed on the outer heating surface circumference runs downwards on the container inner walls into the concentrate supply in the lower part of the container. From the latter is constantly sucked a specific quantity on the lower edge of the outer suction tube, which is transported upwards and sprayed on the overflow edge above the liquid level into the container. Thus, there is already a mixing on the container inner wall, in that the concentrate sprayed on the overflow edge of the outer immersion tube meets the concentrate sprayed by the heating surface and running down the inner container wall. As a result of the constant circulatory movement, there is a homogenization and degassing of the concentrate in the lower part of the container.
Partial or complete circulation operation can most easily be brought about in that the product feed line is connected to the concentrate drain line by means of a bypass with a valve.
A constructionally simple drive coupling between the rotor and outer immersion tube is obtained in that the outer immersion tube immersed in the concentrate is connected by means of stay bolts with the outer rotor circumference.
A similar simple drive coupling for the inner immersion tube is obtained if the latter is connected in the vicinity of the overflow edge by means of stay bolts to a sliding collar or sleeve, which is located on an extension of the drive shaft projecting into the rotor.
The supply and removal of the heat carrier can be constructionally achieved in that the rotor comprises three concentric, conical walls, whereof the axially inner wall forms the heating surface and together with the middle wall defines a heating chamber for the heating medium, whilst the middle wall and the outer wall form a chamber for the return of the heating medium, the heating chamber being connected on the outer circumference of the rotor with the arms of the spider supplying the heating medium and on the inner circumference with the chamber returning the heating medium, which is in turn connected to the corresponding arms of the spider.
In the aforementioned construction the hollow drive shaft, which is known per se from the prior art, and the hollow arms of the spider are used not only for the transmission of the torque to the rotor, but also for supplying and removing the liquid or vaporous heat carrier.
In this rotor construction it is appropriate for the heating surface of the rotor to have between the arms of the spider a draining edge for the concentrate and that in the container, level with the draining edge of the heating surface, there is provided a downwardly and outwardly sloping spray protection means. The latter ensures that the concentrate sprayed on the heating surface outer edge is not splashed within the container and entrained with the vapours and is instead converted on the container wall into a film, which under the action of gravity drains into the lower part. The spray protection means can be formed by a correspondingly inclined part of the container wall.
Advantageously the container has a removable cover or lid, with which the rotor and the immersion tubes are connected by means of the drive shaft and the mounting support thereof and that the cover closes an opening in the container whose diameter is larger than that of the rotor or the outer immersion tube.
In this embodiment the functional parts of the film-type evaporator are fitted to the cover, so that they can be lifted therewith out of the container. Thus, maintenance, repair and cleaning work can be carried out particularly easily.
This objective is also achieved by the fact that the container has a base, which is roughly centrally connected to the product feed line and to which the chamber is fixed and which outside the latter is connected to the concentrate drain line.